DE1945422B2 - Process for the production of silica gel - Google Patents

Description

a) by neutralizing an aqueous sodium silicate solution Silica hydrogel falls and a slurry of the same with an SiOj concentration of 5 to 12% and forms a pH value of 3 to 8, the temperature during the precipitation in the range is kept from 0 to 10 ° C,

b) the product thus obtained is heated at a pH of 3 to 8 and a temperature of 50 to 100 ⁰ C for 1 to 6 hours,

c) the sodium ion concentration in the product thus obtained by washing out and reduced below 20 ppm in the filtrate

d) the product obtained in this way, if appropriate, to a size of the hydrogel particles Crushed below 200 microns in diameter,

characterized in that the gel obtained according to steps a) to d)

e) a water-immiscible solvent which forms an azeotrope with water adds and

f) separating the water from the gel by distilling off the azeotrope and this dries

The invention relates to the production of silica xerogels with a narrow pore diameter distribution Pore diameter range from 300 to 600A and with a surface area in the range from 200 to 500 m * / g.

In the field of silica gels, there is the problem of producing firm, hard gel masses that when Drying does not shrink or shrink and does not tear. The gels are suitable as absorbents and catalyst bases. Gels of small particle size and narrow particle size distribution, with an average diameter of about 50 microns, have been found to be suitable for fluidized bed or stirred reactor catalysis proven best as they are more easily vortexed, less so are strongly subject to degradation through abrasion and particle breakage, and wear and tear on the equipment is reduced is

The only previously described silica gels of suitable particle size and narrow particle size distribution for the above-mentioned purposes have a maximum pore volume of about 1.8 to 1.9 cm ^s / g and a surface area of about 250 to 350 m * / g. Silica gels described in the literature with larger average pore diameters have only been obtained as aerogels. When wet, aerogels are subject to shrinkage due to the coalescence of the particles. This results in a loss of porosity and the products are

for certain uses, in particular as polymerization catalysts, no longer satisfactory.

The earlier German patent 1940068 relates to a Kieselxerogel with a surface area in the range from 200 to 500 m * / g, the pore volume of which is mainly is formed by pores in the range of 300 to 600 Å, which is obtainable by

a) silica hydrogel precipitates by neutralizing an aqueous sodium silicate solution and forms a slurry of the same with an SiO ₂ concentration of 5 to 12% and a pH of 3 to 8, the temperature during the precipitation being in the range from 0 to 10 ° C salary

will be

b) the product thus obtained at a pH of 3 to 8 and a temperature of 50 to 100 ° C heated for 1 to 6 hours,

c) the sodium ion concentration in the so obtained Product reduced by washing out to below 20 ppm in the filtrate,

d) the product obtained in this way, if appropriate, to a size of the hydrolysis particles Crushed below 200 microns in diameter,

e) the hydrogel particles freeze so that the water in the pores is kept in the frozen state will, and

f) the water is removed from the gel by vacuum sublimation

According to the also older German patent 1940093, a silica xerogel with a surface area in the range from 200 to 500 m ² / g, the majority of which are produced by pores in the range from 300 to 600 A, by performing steps a) to d) according to the German Patent 1940068 and then, following these steps, the water is displaced from the product with an organic liquid capable of displacing water and reducing the surface tension in the gel pores, the organic liquid is removed by drying the product and the product is at a temperature in the range Calcined from 260 to 650 ° C

The invention also relates to a process for the production of silica xerogel with a Surface in the range from 200 to 500 mVg, the main part of which is formed by pores in the range from 300 to 600 A.

is formed in which steps a) to d) corresponding to steps a) to d) in the two aforementioned, makes older patents. The method according to the invention is characterized in that that following steps a) to d) to the gel

e) a solvent which is immiscible with the water and forms an azeotrope with water adds and

f) by distilling off the azeotrope, the water is separated from the gel and the gel is dried.

The inventive method is carried out by

a) a silica gel by neutralizing aqueous JrNa ₂ O ySiO ₂ with a strong acid, a weak acid such as CO ₂ , ion exchange resins or in another suitable manner with a

³ 4

stoichiometric proportions with good solvents for the azeotropic destination smd both

^ SJSfS ^^ JT ^ S ^{ehXer SiO} * - ^ ** ¹ "* Benzene, Toluof Xylene, jSjSSl

Kr EndaafiMdilaiiiimBiis of n-propyl formate, n- and isobutyl formate, n- and

Ι ίο.? ΑκΓ ^ ΖΤΓ ^about isoamylformate, benzylformmt, n-propyl acetate, n-

8 to 9'h, works up to 40 «Vo of the required 5 and isobutyl acetate, n- and isoamyiacSat Benzyl-

! S ¹ LSJS ?? ^ XS "™ ^{m 30 to 8} ^ phenyl acetate, ethyl propionic n-Propylpfo-120, preferably about 60 minutes, and the pionate, isobutyl propionate, isoamyl propionate, MeÜiyl ·« sting 60 · /. M further about 20 to 90, vor- butyral n-Propy7utjS TÄÄl ^^ u ^Va ^ f ^ T ^a A ⁴⁵ JS ⁰ " ¹⁶¹¹ . ² " ⁵ ^ the isoamyl butyrate, methyl isobutylate, ethyl isobutyrate, temperature during the filling in the range of io diisobutyl ether, diisoamyl ether, diphenyl ether, from about 0 to 10, preferably to about 5 ° C. -Value of the precipitates immiscible with water and during the distillation development of 3 to 8, preferably about 5 to 7, working with water to form azeotropic mixtures, medium ^ö ^

b) the suspension of the precipitated silica hydro- 15 A preferred method for carrying out this gel at a pH of 3 to 8, the preferred distillation process is illustrated in FIG. The wise 5 to 7, and a temperature of 50 to 50 to hydrogel is entered here through valve A into the around 100, preferably about 90 ⁰ C, 1 to 6, preferably jacketed container 1, in which further wise about 4 hours , heated, _m collection tank 3, mk water not miscfa-

c) the product of a washing treatment with on ao bare and with water an azeotrope-forming Lö-Natriuni displacing salts is pumped in such a solvent. Preferably works subjects the sodium concentration to one of the amounts of water in container 1 to be entrapped of the Fdtrats aegt below 20 ppm, whereby to be mentioned amount of solvent, albeit also suitable salts aluminum chloride or sulfate smaller amounts can be used. The container 1 and ammonium chloride or nitrate and Mi- »5 is then heated

loops of the same belong and with the The distillate is via the heat exchanger 2

urination also gained by washing with strong in the collecting tank 3, in which the azeo

Acid, such as hydrochloric acid or sulfuric acid, takes place in two phases. The water is separated

can, decanting separated

d) and optionally the gel particle size 30. The solvent fraction is recirculated by continuously shear-sharp mixing of the material from liches pumping back into the container 1 in the stage c to obtain particles with a system, and the system is 2 to 20, mean diameter of operated below about 200 microns, preferably about 5 hours, after which one crowns and preferably in the range from SO to the pump 4 is stopped and excess solution-150 microns reduced ₃₅ medium distilled off until the silica gel has dried.

At the bottom of the tank 3 are two high or Then low-capacitance probes 5, 6 are arranged which

control the motor valve 7 through which the water

e) a water immiscible, run with water from the tank via the pipe 8 an azeotrope-forming 40 during the distillation.

Solvent is added and the dry silica gel is passed through valve B

f) the azeotrope is distilled off and discharged in this way.

essentially all of the water removed. The properties of the gel according to the invention are then dried by distilling off the silica xerogels and especially the excess porosity solvents average if calcined, with temperatures in the pore diameter (Pd), with range from 260 to 650 ⁰ C and preferably
about 540 ⁰ C can be used. 4PV-10 *

50 Pd =

Neutralized in the case of the Kieselhydrogelfälhmg in stage a) one preferably with a strong acid.

To carry out the azeotropic distillation, the following applies: and the pore size distribution is specified. the A solvent is added to the wet gel to determine the values of the various properties in such an amount that an excess of more than 55 shafts occurs according to that described in the literature Nitrogen absorption-desorption technology researched for the azeotropic removal of water. the same amount is available. The Azeotropic Mixture Examples 1 and 2 describe a preferred one is then preferably alternating between a total of 5 to 20 hours of working to carry out the process half an hour to two hours reflux of the invention to obtain a pebble treated and distilled. 60 xerogels with the properties mentioned, while a further advantageous procedure for which the other examples differ in their critical nature the azeotropic distillation consists of explaining process conditions or suitable ones in that the solvent in step e) in alternative procedures for the preparation of the Kieseleinem show substantial deficit below the BiI- xerogels, To make an azeotropic mixture, the quantities required 65 Example 1 adds and the solvent after decanting the

Water from the distilled off azeotropic mixture. 20 160 g of sodium silicate solution are added with a

continuously recirculates. Suitable content of 28.7%> of SiO ₂ and 8.9 · / · of Na ₂ O too

25 440 g of water are added and the mixture cools to 5 ° C with agitation.

21,305 g of H ₂ SO ₄ (12.75 percent by weight) are then added, namely 8520 g in one hour and the remainder in 45 minutes. The final pH value of the precipitate is 5.0.

The slurry is then heated to 95 ° C. over 2 hours and held at this temperature for 2 hours _{and washed with a solution of 2225 g of NH 1} NO ₃ in 1701 of water and then with deionized water until the filtrate is titrimetrically below 20 yields ppm Na ₂ SO ₄

The product is then homogenized for 30 minutes, after which a 300 g sample is withdrawn and settled leaves and decanted the excess water.

60 g of 11% by weight slurry are recovered Solids content and add them to 1500 g of ethyl acetate in a still. That Azeotrope is then distilled off at atmospheric pressure by working for a total of of 8 hours alternately for half an hour distilled and half an hour with reflux treatment. As a precautionary measure the liquid level in the flask is maintained by continuously adding solvent. The temperature fluctuations during the azeotropic distillation are 70.5 and 71 ° C. After The gel is dried for 8 hours by distilling off the remaining solvent.

The silica xerogel obtained is calcined in an oven at 540 ° C. for 4 hours before the evaluation.

The analysis of the physical properties results in SA 307 mVg, PV 2.35 cmVg and Pd 317 A.

Example 2

A 1000 g sample of the material present after homogenization in Example 1 is withdrawn and allowed to settle and decant the excess water.

A 200 g sample of the 11 weight percent slurry The solids content is then measured in a laboratory apparatus set-up corresponding to FIG. 1 Arranged 2000 ml distillation flask entered, into which one is placed before the start of the distillation further pumps in 1000 g of ethyl acetate; the distillation is carried out for 5 hours. That continuously Recovered ethyl acetate is placed in the still after separating the water by decanting pumped back with the distillation of the solvent phase at a corresponding speed.

After 5 hours, the pump is switched off and the excess is removed to obtain a dried silica gel Solvent distilled off.

The xerogel obtained is calcined in an oven at 537.7 ° C. for 4 hours and then calcined to its physical properties Properties assessed SA 323 m * / g, PV 2.53 cmtyg, Pd 313 A.

Fig. 2 shows a pore size distribution curve that the majority of the pore volume of the Product silica xerogek on the gel with a pore size distribution in the narrow range of 300 to 600 A decreases

Calcmienmg for 3 hours in a fluidized bed at 982 ° C does not change the physical properties: Sa 329 mVg, PV 2.47 cm »/ g, Pd 301A.

Example 3

A 300 g sample is taken after homogenization in Example 1 from the present good and decanted the excess water.

A 60 g sample of 11 weight percent solids is then added to 1500 g of ethyl acetate. The azeotrope and then the excess solvent are continuously distilled off

The resulting Kieselxerogel is 4 hours in the oven

at 540 ⁰ C and calcined to its physical

Properties assessed: SA342mVg, PV 1.91 cm ^s / g,

Pd 223 A.

As this experiment shows, when applied

ία an azeotropic distillation without using the The pore volume and conditions described in Example 1 or 2 for drying the product accordingly adversely affects the average pore diameter.

Example 4

With the exception of the silicon dioxide concentration, all variables are retained during the precipitation according to Example 1; as SiO ₂ -end-

ao concentration 7 and 10% are chosen. The effect of the SiO ₂ concentration on the porosity of the gel immediately after the precipitation is shown in FIG. 3.
Like Fi g. 3 shows, in a silicon dioxide

a5 final concentration in the slurry of 7 or 10 percent by weight, respectively, relatively smaller pore volumes than in example 1, in which an SiO ₂ concentration of about 8.5 percent by weight is established. As FIG. 3 further shows, the surface area increases with the SiOj concentration. In order to achieve a maximum pore volume and at the same time a good surface area, it is therefore preferable _{to keep the SiO 2} concentration between approximately 8 and 9 percent by weight, particularly preferably at approximately 8.5 percent by weight.

Example 5

With the exception of the rate of acid addition, all variables are retained during the precipitation according to Example 1. The acid portion corresponding to 40% of the stoichiometric amount, based on SiO ₂ , is added in 0.5 or 2.0 hours.

As the present example in Fig. 4 clearly shows, both the pore volume and the surface depend on the speed with which the neutralization medium, in the present case sulfuric acid, is added to the aqueous silicate solution. As the results show, one Addition of 40% of the stoichiometric amount in about 1 hour. Optimal and faster or slower In addition, lower pore volume and surface area values obtain.

Example 6

With the exception of the temperature, all variables are during the precipitation according to the example 1 retained. Precipitation occurs at 25

or 50 ° C

Like F i g. 5 shows, the pore volume increases with increasing Falling temperature Auen the surface shows the tendency with increasing fall temperature decrease, with the curve of this parameter itself

but at about 5 ° C it approaches the horizontal course. This temperature stiffens the preferred fall temperature to ensure optimal pore volume and surface values.

1533

Example 7

With the exception of the use of CO ₂ to neutralize the sodium silicate instead of sulfuric acid, all variables are retained during the S precipitation according to Example 1. The physical properties of the silica gel are similar to those obtained in Example 1.

Example 8

All variables during the precipitation are retained as in Example 1. After the felling the gel is made by adding 2N NaOH and heating to pH 9.0 for one hour set, whereupon the gel is washed with water and acetone as in Example 2 before evaluation.

The silica gel gives SA 346 mVg, PV 1.96 cmVg, Pd 226 A.

As in the comparison of the properties of the gel obtained according to Example 9 with a product according to ao Example 1 shows that too high a pH and relatively short heating leads to aging a loss of pore volume. In Example 1, the pore volume is 2.35 emtyg, in the present case Example received only 1.96 emtyg. »5

Example 9

All variables during the precipitation are again retained as in Example 1. After precipitation, the gel is _{adjusted to a pH value of 5.0 by adding 1 nH SO 4} and heated to 95 ° C for 4 hours. Samples are taken at various times and washed with water and then acetone before evaluation as in Example 4.

The following shows the influence of the heating time on the porosity of the gel and its pore distribution Tabel.

Time SA PV Pd Snd. mVg cm '/ g A. 0 757 2.77 147 Vi 492 2.72 221 1 394 2.90 294 1 to Vt 355 2.76 311 2 343 2.77 323 4th 284 2.56 362

As the table values show, the thermal aliening of the precipitation product is intended to achieve a Product silica xerogels having an average pore diameter in the desired range of about 300 to 600 A for at least about 1 hour.

Example 10

All variables during precipitation and heating are again retained as in Example 1. The gel is then washed as in Example 1, but only down to a titrimetric Na ₂ SO ₄ content of the filtrate of 100 ppm, whereupon the product is treated with acetone and evaluated. The physical properties of the gel correspond essentially to those obtained in Example 1.

If the product is calcined and evaluated for 3 hours in a fluidized bed at 996 ° C., SA 225 m ² / g, PV 1.62 cmVg and Pd 286 A are obtained.

As the porosity values of the product obtained here show, the surface, the pore volume and the average pore diameter are due to the relatively high de! Product of Na ₂ SO ₄ adversely affected.

For this purpose 2 sheets of drawings

509529,

1533

Claims (1)

Claim: Process for the production of silica gel nut a surface area in the range from 200 to 50 m * / g, the pore volume of which is mainly is formed by pores in the range from 300 to 600 A, in which one